1. Field of Invention
This invention relates to fiber optic applications and more specifically, to an adiabatic bend transducer to support higher-order mode filtering and/or fundamental mode amplification for passive and active fiber assemblies.
2. Description of Related Art
Rare-earth doped fiber amplifiers offer important advantages compared to solid-state lasers in terms of power conversion, transverse mode stability, compactness and thermal management. See, e.g., A. Tünnerman et al., “The renaissance and bright future of fibre lasers,” J. Phys. B 38, pp. 681-693 (2005), the disclosure of which is incorporated by reference herein in its entirety. A double cladding architecture and enlarged fiber cores allow the amplification of optical signals up to power levels appropriate for laser processing such as micro-machining and laser marking. See E. Snitzer et al., “Double-clad, offset core Nd fiber laser,” Optical Fiber Sensors, OSA Technical Digest Series, vol. 2, postdeadline paper PD5 (1988), the disclosure of which is incorporated by reference herein in its entirety. In that way, even chirped pulse amplification to generate femtosecond and picosecond pulses can be achieved. See U.S. Pat. No. 5,499,134 to Galvanauskas et al., the disclosure of which is incorporated by reference herein in its entirety.
However, nonlinear effects (namely stimulated Brillouin scattering, stimulated Raman scattering, and/or self-phase modulation) limit the power scaling of fiber amplifiers to higher power levels. This is commonly overcome by further increasing the core diameter while decreasing the fiber core numerical aperture down to the technical limitations of 0.06, which results in a large mode area (LMA) fiber. In order to maintain an only diffraction limited output beam, single-mode operation of the fiber amplifier is highly desirable. This limits the effective fiber core area to a certain level. An increase beyond this level will give rise to higher order modes and thus result in a multimode LMA fiber that requires mode suppression techniques in order to preserve nearly fundamental mode propagation and thus the beam quality.
The most commonly used mode suppression technique is bending the fiber. Bending will induce a higher loss to the higher order modes than the fundamental mode, a characteristic which is typically referred to as “bend loss.” Coils are required in any way, since the doped fiber will usually have several meters of length and packaging the amplifier to smaller sizes requires wrapping the fiber to some extent. The most direct approach is a helical coil achieved by wrapping the fiber around one or two rods of constant radius. See U.S. Pat. No. 6,496,301 to Koplow et al., the disclosure of which is incorporated by reference herein in its entirety. Fiber bends not only induce pure bend loss to the modes, but also inflict a transition loss at the transition from a straight fiber to a bend section and vice versa. See E. A. Marcatili et al., “Improved Relations Describing Directional Control in Electromagnetic Wave Guidance,” Bell Sys. Tech. J. 48, pp. 2161-2188 (1969), the disclosure of which is incorporated by reference herein in its entirety. A bend transition in a multimode fiber will couple light out of the core as well as inflict coupling between guided core modes. In the case of the aforementioned patent to Koplow, light will be coupled to unwanted higher order modes at the beginning and the end of such a coil. See J. Baggett, “Bending losses in large mode area holey fibres,” PhD Thesis, Chapter 2.2.1, University of Southampton (2004). LMA fibers of larger core size are even more sensitive to such changes in curvature, as stressed by recent research. See Sévigny et al., “Modal sensitivity analysis for singlemode operation in largemode area fiber,” Proc. SPIE 6873, 68730A (2008), the disclosure of which is incorporated by reference herein in its entirety. In that way, a helical coil resembles only a compromise between bend induced mode coupling and higher order mode discrimination, and requires improvements.
The transition loss as such is highly underestimated by most people regarding LMA fibers, and has a huge impact on the performance when trying to reach smaller bend radii. This problem could be solved in a better way by adiabatic bend transitions, which change the level of curvature over a sufficient long length of fiber. In that way, transition losses from the fundamental mode will be minimized. See J. Baggett, PhD thesis, supra. The importance of adiabatic transitions in mode filters was further stressed in U.S. Patent Application Publication Nos. 2005/041702 to Fermann et al. and U.S. Patent Application Publication No. 2008/056656 to Dong et al., the disclosures of which are incorporated by reference herein in their entirety. Adiabaticity criterions were recently determined analytically for single mode fibers and computer simulations were performed on transition losses of LMA fibers. See J. Love et al., “Bend Loss, Tapering, and Cladding-Mode Coupling in Single-Mode Fibers,” IEEE Photonic Tech L 19/16, pp. 1257-1259 (2007) and Hadley et al., “Bent-waveguide modeling of large-mode-area, double-clad fibers for high-power lasers”, Proc. SPIE 6102, 61021S (2006), the disclosures of which are incorporated by reference herein in their entirety. Bend limiters exist to thwart tight bends in order to prevent power loss and physical damage of single mode or telecommunication fibers.
The shortcoming of these considerations lies in seeing the transition loss as a simple power loss while neglecting mode coupling, which is inadequate for devices where the mode distribution is important. Little research has been performed on bend-induced mode coupling and very little, if any, research has involved the investigation of bend induced mode coupling for adiabatic bends in LMA fibers. See Laperle, “Yb-Doped LMA Triple-Clad Fiber for Power Amplifiers,” Proc. SPIE 6453, 645308 (2007), the disclosure of which is incorporated by reference herein in its entirety. This effect could further be used to couple light from higher order modes back into the fundamental mode. In that way, adiabatic mode scramblers can be realized to couple unwanted higher order core modes partially into the fundamental mode by series of varying adiabatic bends. Up to now, there are neither criteria for adiabatic bends in multimode fibers, nor precise embodiments.
Another design aspect for a fiber amplifier is the gain distribution and gain competition in the pumped fiber amplifier. The pump light and hence the amplification is stronger on the pumped side of the fiber. Imperfection in real fiber amplifiers, e.g., due to fiber splicing, provokes the generation of higher order modes, thereby decreasing the mode quality at an early stage. The aspect of mode competition in fiber amplifiers is well known. See J. Sousa et al., “Multimode Er-doped fiber for single-transverse-mode amplification,” Appl. Phys. Lett. 74/11, pp. 1528-1531 (1999), the disclosure of which is incorporated by reference herein in its entirety. Fundamental mode excitation further reduces higher order modes caused by amplified spontaneous emission (ASE). See U.S. Pat. No. 5,187,759 to DiGiovanni, the disclosure of which is incorporated by reference herein in its entirety. Thus, higher order modes should be minimized in the shortest length of fiber possible for forward-pumped fiber amplifiers, since they will decrease the gain for the fundamental mode, and they were amplified themselves. This fact is missing in prior analyses, and should especially be implemented into forward pumped fiber amplifier designs for improved operation.
The last design consideration involves the strong reduction of the mode area in LMA fibers by bending. See J. Fini, “Bend-compensated design of large-mode-area fibers”, Opt. Lett. 31, pp. 1963-1965 (2006), the disclosure of which is incorporated by reference herein in its entirety. For a forward pumped fiber amplifier, the signal power density increases towards the end of the fiber, which makes further increases by a reduced mode area critical. This implicates a reduction of the bend of the mode filter towards the end of the amplifier to avoid reaching the non-linear effect region. On top of that, a tight bending at the beginning induces a significant loss to the fundamental mode as well, which makes a reduction of the bend radius inevitable. A constantly alternating bend structure provides the best solution for this, which results in the shape of a spiral towards the amplifier output. A spiral would additionally provide the smoothest bend transition and such meet the aforementioned adiabaticity criterion.
So far, only a few attempts have been attempted to reduce the mode coupling at the beginning and the end of a coiled fiber amplifier. Even fewer consider a non-symmetrical bend design, e.g., U.S. Patent Application Publication No. 2009/059,352 to Fini, the disclosure of which is incorporated by reference herein in its entirety, and none of the attempts so far takes into account the non-uniform distribution of the gain along a forward pumped amplifier in combination with adiabatic bend transitions. It is believed that no other preliminary work has been done to design an optimized adiabatic fiber bend transducer.